Geologic formations are used for many purposes such as hydrocarbon and water production. Boreholes are typically drilled into the earth in order to intersect and access the formations.
Electrical earth borehole logging is well known and various device and various techniques have been described for this purpose. Properties of the formation may be estimated by transmitting an electromagnetic (EM) wave in the formation, and receiving related signals at the receiver antennas. Logging instruments may be used to determine the permittivity and electrical conductivity of an earth formation.
Electromagnetic induction resistivity instruments can be used to determine the electrical conductivity of earth formations surrounding a wellbore. These instruments may be outfitted with antennas that are operable as both transmitters and receivers. Transmitter antennas and receiver antennas may have the same characteristics, and an antenna (e.g., coil) may be used as a transmitter at one time and as a receiver at another.
In transient electromagnetic (TEM) methods, a transmitter is configured to initiate currents in surrounding earth formations. The transmitter and receiver may be at the surface or within the wellbore. Induced electric eddy currents diffuse outwards from the proximity of the transmitter into the surrounding formation. At different times related to the excitation transient, information arrives at the measurement sensor (e.g., an induction antenna) predominantly from different investigation depths. Generally, early-time signals predominantly relate to near-zone responses (lesser depths of investigation) and late-time signals predominantly relate to remote-zone responses (greater depths of investigation).